Example value as null fails

Describe the bug When OpenAPI specification contains null as an example value the Spectral engine reports Cannot read properties of null (reading 'type')

To Reproduce

Run Spectral validation on the following OAS:

openapi: 3.0.0
info:
  version: 1.0.0
  title: Cat
  description: Cat API
  contact:
    name: Cat owner
    email: cats@cat.com
    url: https://developer.cats.com/contact-us
  license:
    url: https://www.cats.com/api-license-terms
    name: Cats 1.0
servers:
  - url: https://api.maersk.com/products
    description: Production Environment
paths:
  /cats:
    get:
      operationId: get-cats
      description: Cats
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                type: string
                nullable: true
                example: null

Expected behavior It is expected that null is an allowed example value for nullable properties.

CLI Environment:

Spectral CLI version: 6.11.1
OS: Mac

CLI Environment Output

(node:46129) [DEP0040] DeprecationWarning: The `punycode` module is deprecated. Please use a userland alternative instead.
(Use `node --trace-deprecation ...` to show where the warning was created)
Error running Spectral!
Use --verbose flag to print the error stack.
Error #1: Cannot read properties of null (reading 'type')

Node.js Environment

@stoplight/spectral-core 1.18.3
@stoplight/spectral-parsers 1.0.3
@stoplight/spectral-ruleset-bundler 1.5.2
@stoplight/spectral-rulesets 1.19.0
@stoplight/spectral-runtime 1.1.2

Node.js Environment Output

stoplightio / spectral

Example value as null fails #2649